1. Field of the Invention
The present invention relates to the use of surface stabilized ferroelectric liquid crystals (SSFLC) for electro-optic light valves having one of two stable states. In particular, the present invention relates to using SSFLCs for electro-optic shift registers.
2. Description of the Related Art
Liquid crystal devices (LCD) have found prominent use in non-emissive electro-optic display applications because of their unique characteristics including low voltage and low power operation. These applications include the development of flat-panel displays used in computer terminals, oscilloscopes, radar and television screens.
Improvements have been made to provide a faster, polarity sensitive electro-optical device. For example, U.S. Pat. No. 4,367,924, issued to Clark et al. discloses an electro-optical device which utilizes the strong coupling of the molecular orientation of the liquid crystal to an applied electric field. Tilted chiral smectic liquid crystal material is disposed between flat plates spaced by a sufficiently small distance to ensure unwinding of the helix formed by the molecules of the liquid crystal in bulk. As a result, the liquid crystal assumes one of two stable states which can be switched by reversing the polarity of an externally applied electric field, thus forming a bistable light valve. This type of bistable light valve, which employs surface interactions to stably unwind the spontaneous ferroelectric helix, is known as a Surface Stabilized Ferroelectric Liquid Crystal (SSFLC) device.
Such a bistable light valve has been used to develop electro-optic shift registers which propagate data through an LCD array. For example, U.S. Pat. No. 4,815,035 issued to Brooks discloses a scrolling liquid crystal light modulator having an LCD array having different optical states propagated by a plurality of electrodes located along the array. The LCD cells each have three bias levels having three distinct functions: a high bias level which switches the LCD cell to the ON state; a low bias level which immediately extinguishes the ON state (e.g., to an OFF state); and an intermediate bias level which results in an ON state in the LCD cell only if the immediately preceding adjacent cell is in the ON state. Thus, the intermediate bias level permits propagation of data from one cell to the next adjacent cell.
FIGS. 1A to 1F correspond to FIGS. 2A to 2F of U.S. Pat. No. 4,815,035. As shown in FIG. 1A, an input liquid crystal cell assumes an assigned optical state (ON) during Phase 1 in response to the input signal (H) applied to electrode 16. The assigned optical state of the input liquid crystal cell is then propagated by applying to the subsequent LCD cells a three-phase pulse sequence. Specifically, during Phase 2 as shown in FIG. 1 the electrode 18a adjacent to the input electrode 16 switches to a medium bias (M) so that the optical state (ON) is propagated to cover both electrodes 16 and 18a. During Phase 3 as shown in FIG. 1C the input electrode 16 applies a second input signal (L) which extinguishes the input liquid crystal cell to another optical state (OFF); also, the next adjacent electrode 18b switches from a low bias (L) to a medium bias (M) to propagate the optical state (ON) to the LCD cell affected by the electrode 18b The three phase cycle thereafter repeats itself, so that data represented by the optical states can be propagated along the LCD array.
Although U.S. Pat. No. 4,815,035 to Brooks discusses in general electro-optic shift registers, Brooks fails to teach any specific relationship between the electrode spacing, the cell thickness and electromagnetic fields within the cell; thus, these factors associated with domain wall propagation must be more carefully considered so that those of ordinary skill in the art may consistently create LCD cells in which domain walls are accurately and reliably propagated.